how to change the background color of a frame (or this is mission impossible on gtk)

[Ofer Oshri <ofer oshri xeround com>]

hi,
i am trying to port a qt application to gtkmm.  the trivial task of setting the color of a frame became a mission impossible task.  here is a simple example of what i am trying to do (on fc6 + kde):

///////////////////////////////////////////////////////////
#include <gtkmm.h>

class window_with_frame : public Gtk::Window {
protected:
    Gtk::Frame frame_;

public:
    window_with_frame()
    {
        set_title("frame example");
        set_size_request(300, 300);
        set_border_width(10);

        // change the color of the frame...
        Gdk::Color cl("red");
   
     frame_.modify_bg(Gtk::STATE_NORMAL, cl);

        add(frame_);
        frame_.set_label("frame");
        frame_.set_shadow_type(Gtk::SHADOW_ETCHED_OUT);
        show_all_children();
    }
    virtual ~window_with_frame() {}
};

int main(int a, char *b[])
{
    Gtk::Main kit(argc, argv);
    window_with_frame window;
    Gtk::Main::run(window);
    return 0;
}

g++ f.cpp -o f `pkg-config gtkmm-2.4 --cflags --libs`
///////////////////////////////////////////////////////////

any advice/help/reference will be appreciated.
ofer.